Abrasive structures and method of making



A. E. RAYMON D ABRASIVE STRUCTURES AND METHOD OF' MAKING March 3, 1959 Filed June 2l, 1954 W W N a ,M f M Y W 6 m M y M W .MM :Mk m

United States Patent O ABRASIVE STRUCTURES AND METHOD F MAKING Albert E. Raymond, St. Paul, Minn., asslguor to Minnesota Mining & Manufacturing Company, St. Paul, Minn., a corporation of Delaware Application .lune 21, 1954, Serial No. 438,250

12 Claims. (Cl. 51298) The present invention relates generally to the field of coated abrasives. More particularly, my new and novel invention pertains to flexible coated abrasive sheet structures having uniformly spaced, oriented abrasive particles and to methods for making the same.

Heretofore, flexible coated abrasive sheets have been made principally by randomly applying abrasive particles on an adhesive coated backing, drying or curing the adhesive and then further strengthening the bond of the particles to the sheet by a second adhesive coat known as a sand size or sizing coat. Generally, when an abrasive sheet so prepared is to be used for operations where high abrading efficiencies are desired, as many abrasive particles are deposited on the backing as may be securely bonded thereto by the successive adhesive bonding and sizing coats. The abrasive particles for the most part lie in a more orless haphazard fashion against and atop one another in clusters, layers and groups, and extend out varying distances from the backing sheet due to both the haphazard arrangement of the layers and clusters of abrasive particles and to'variations inlength and size thereof. Even when the particles are applied and partially oriented by known electrostatic and electromagnetic coating procedures, the haphazard grouping effect to a lesser degree is present. y

When an abrasive sheet so prepared is used for normal abrading operations, the instantaneous rateof cut varies over the life 'of the sheet, generally decreasing gradually as the sheet -is used from a high initial rate to a rate below economic usefulness. This gradual decrease in rate is primarily due to the'manner in which the abrasive particles become abrasively effective. Initially', the effective abrasive particles are those extending furthest from the backing, and which first come into contact with the work. As these initially effective particles become dulled, frictional forces exerted thereupon by the work increase, and one by one the abrasive particles are either broken off or, upon failure of the grit bond, are wrenched free of the sheet. As numerous ofvthese further extending particles are removed, the abrading plane is lowered and abrasive particles become effective which did not extend from the backing as far as the initially effective particles.

As the abrading plane more nearly approaches the backing sheet. abrasively effective particles become more and more numerous. Without increasing the pressure of the abrasive surface against the work, the pressure per effective abrasive particle decreases; since cutting rate per effective particle varies with pressure, the rate dwindles. Subsequently, when the pressure on the effective particles decreases to the extent that substantially no more stock is removed and further use of the sheet merely further dulls the particles, the `recognizable "glazing over of the abrasive sheet occurs. The time for dulling and resultant glazing is hastened by groups of abrasive particles which collectively form dulled plateaus" or at areas, having negligible, in fact negative, abrasive effect.

ICC

Theflglazing occurs` when there is yet a substantial portion of abrasive particles remaining and, consequent ly, the sheet is rendered commercially useless while the backing is still in effective condition and before a com- .abrasively coated areas.

plete loss of abrasive particlesoccurs. I have found that in many metal abrading operations glazing occurs when at least 25 percent of the orginal abrasive mineral remains. Not only is an excessive number of abrasive sheets necessary for a job accompanied by corresponding high material costs, but extra unnecessary time is consumed during the replacing of the old useless sheet with ar fresh one.

Even in the type of abrasive sheets known as open coat" sandpaper, many of the abrasive particles lean against and atop one another in a manner similar to that above described. In this type of sheet, the abrasive particles are not in layers and the number applied is insufficient to cover the entire surface of the backing sheet, thereby leaving a considerable portion of the surface uncovered in the form of :irregularly shaped open spaces between the groups andclusters of grains. Open coat abrasive sheets, which generally are not suitable for abradingpoperations requiring high cutting rates, have utility inrefinishing operations where abrasively removed materials, e. g. paints and varnishes, tend to fill and clog the sheet. These abrasive sheets also will glaze over, prematurely, necessitating replacement before the abrasive mineral has been substantially completely utilized. v

Abrasive sheets have been suggested having intermittent spaces thereon full-coated with layers of many abrasive particles separated by comparatively large non- Although there are some advantages obtained with this type, the sheet will be pre maturely inetfectuated by glazing and further, will not exhibit a uniform instantaneous rate throughout the life of the sheet.

l have found if the abrasive particles in a flexible coated abrasive sheet are individually separately located' on the sheet in spaced relation to one another with regularly shaped open spaces therebetween, the individual particles will be more securely attached to the sheet, thus will individually and collectively be effective for a longer period and the sheet will be effective in commercial" operations until substantially all of the abrasive particles are removed and/or the backing wears out, becomes soggy and breaks up. A

Therefore, an object of my new and novel invention is to provide a flexible coated abrasive sheet wherein substantially all the abrasive particles are utilized effectively in abrading operations during the useful life of the sheet, thus eliminating necessity of replacement before the, abrasive coating is exhausted.

A further object of the present invention is to provide an abrasive sheet which is versatile and useful for finishing and refinishing operations.

A still further object of the present invention is to provide a coated abrasive sheet whose abrasive cutting rate remains substantially uniform throughout the abrading life of the sheet.

'Still another object of my novel invention is to provide a method for producing my coated abrasive sheet having substantially all the abrasive particles regularly spaced with the long axes thereof perpendicular to the backing sheet with regularly shaped spaces between the individual particles.

These and other objects and advantages will be pointed out, or will be apparent from the specification and attached drawingstaken as wa whole, including the appended claims.

Referring now to the accompanying drawings, Figures 1, are schematic diagrams of apparatus which may be used for carrying out methods according to this invention and to provide the novel abrasive sheet product claimed hereinafter.

Figures 2 and 3 are plan views of alternative types of portions of the apparatus of Figure 1, and Figure 4 is a perspective view of a portion of the composite'fabrasive structure obtained.

Figure 6 is a reproduction of a chart showing how abrasive sheets having varying numbers of individually spaced oriented abrasive particles per unit area varies in total abrasive effectiveness.

` Figure 7 represents in enlarged cross-section a modification of my regularly individually spaced oriented abrasive sheet material.

Referring to Figure l, a backing web 10 unwound from a supply roll 11 is led through adhesive applying rollers 12 which coat o'ne side of the backing web with a continuous layer of bonding adhesive 13 contained in trough 14. After the web is passed over suitable guide rollers and aftertraveling a distance at room temperature, i. e., about 25 C., such that a portion of the solvent contained in the adhesive evaporates from the surface thereof leaving a partially dried film or skin on the surface, the web comes in contact with the masking web 15. The masking web, containing regularly spaced apertures therein, is unwound from supply roll 16 and laminated in superimposed position over contact roll 17 on the adhesive face side of the backing web 10 at a contact pressure insufficient to rupture the adhesive skin.

' After being thus joined, the composite web of the coated backing and the masking web is led under mineral hopper 18 (which may be fitted with an adjustable gate or outlet, not shown, to control the quantity of particles released) where loose excess abrasive mineral particles 19 fall freely by gravity onto the composite. Many of the loose particles fall or are jostled into position in the apertures of the masking web by the action of a beater 20 on the composite. The sharp abrasive points and edges of the mineral particles pierce the skin of the partially dried adhesive film and become suiciently anchored therein such that substantially none of the embedded mineral particles become dislodged as the combined web is led around drum 21. Most of the excess mineral particles not adhered to the adhesive film fall into receiving hopper 22, and the remaining excess mineral is shaken into receiving hopper 22 when the web is beaten by the action of beater 23. The web then passes around guide roller 29.

The masking web 15 is led over pullover drum 24 and wound on roll 25, thereby being stripped away from the composite sheet material 26. During the removal of the masking web the abrasive particles are straightened such that the longitudinal axes of the abrasive particles are oriented substantially parallel with each other and substantially normal to the backing. The composite sheet material 26 is then suspended on racks 31 in oven 30 during the remainder of the drying or curing'operation and the dried cured sheet 27 is wound up on storage roll 28 or directly sized and cured.

Generally, abrasive particles which are most effective in exible coated abrasive sheets are those shaped so as to have one dimension substantially larger than other dimensions, i. e. the particles are elongated. Herein, abrasive particles of this type are referred to as orientable abrasive particles.

Many attempts have heretofore been made to orient each of the abrasive particles to a position where the longitudinal axes of the particles are normal to the backing sheet. Such a result is partially accomplished when the particles are deposited on the backing sheet by electrostatic or electromagnetic coating methods. When such means are used, many of the particles lie in an oriented position but, as previously mentioned, many of the particles cling and adhere together in disadvantageous clnsters.

I have, by the means and methods herein described, produced a flexible coated abrasive sheet in which the abrasive particles are not only oriented with the long axes thereof normal to the backing, but also which exhibits the additional feature of having each abrasive particle individually separately regularly spaced, a heretofore unattainable product. Further, I am able to produce an abrasive sheet wherein the particles remain oriented and individzally regularly spaced throughout the life of thc sheet.

Referring to Figure 2, the masking web 15 may consist of longitudinal filaments 32 with transverse filaments 33 woven therewith in the form of a cloth to produce restricted abrasive coating areas when the masking web 15 is superimposed on adhesively coated backing web 1li.

Figure 3 is a plan View of a portion of an alternative type masking web 15', consisting of a suitable continuous ilm 35 with spaced portions removed to provide apertures 36 resulting in restricted space abrasive coating areas when the web 35 is superimposed on adhesively coated backing web 10. When speaking collectively of holes or interstices in the various types of masking webs, l refer to them as apertures.

Figure 4 illustrates a perspective view of an area of cured composite abrasive coated material 27 after the stripping away of previously superimposed masking web 15, wherein is shown individually regularly spaced oriented abrasive grains 19 adherently attached to the adhesive binder 13 coated on backing l0 and which have been individually deposited in the restricted abrasive coating areas. Although the cured sheet 27 will be suitable for some operations without a sizing coat, such a coat will generally be applied and cured in a manner well known to the art prior to use for most abrading operations.

The pattern formed by the individually spaced abrasive particles will correspond to the apertures of previously superimposed masking web 15 now removed. Generally, the abrasive particles will be substantially equidistant from one another, i. e. the distances between the individually spaced particles forming a row laterally of the machine direction of the web will be the same'as the distance between that row and the preceding and following rows. Successive rows may either be in register (such that abrasive particles of each lateral row form series of rows in the machine direction as well as laterally thereof) or out of register some fixed portion of the distance between lateral particles (in which case the particles of alternate lateral rows form a series of rows in the machine diretion). Or I may choose to have the lateral rows at varying distances with preceding and following lateral rows and either in or out of register therewith.

For certain straight-abrading operation, e. g., stripscouring steel wherein scale is removed preceding rolling operations, abrasive sheets with the lateral rows of abrasive particles in register with preceding and following lateral rows tend to leave longitudinal parallel scratch grooves in the steel. Such an occurrence is called ridging. I substantially eliminate this ridging effect by utilizing abrasive sheets wherein the lateral rows of vparticles are out of register with preceding and following rows; however, I generally prefer to repeat the pattern so at least every fourth lateral row is in register, for little further diminshment of ridging is obtained by repetition at intervals greater than every fourth lateral row and fabrication of the masking web becomes more difficult as thc complexity of the pattern is increased, When the abrasive particlesare placed in the foregoing manner, the several spaces between the abrasive particles are themselves regularly shaped.

I nd particular advantage in the regularly individually spaced oriented abrasive particles with regularly shaped spaces therebetween such as is a part of thepresent invention. For example, material costs of vthe present sive sheets prepared by hitherto known means and I find my sheet is commercially utilizable for long periods before the sheet is exhausted. Time normally consumed in changing sanding discs or belts is saved by using the abrasive sheet herein described thereby increasing available operating time perday or hour. The abrasive particles are individually more completely utilized. I find that an abrasive sheet having regularly spaced oriented abrasive particles may be economically utilized in abrading operations until substantially all of the abrasive mineral has been removed and/or the backing becomes ragged and worn out; in my abrasive sheet glazing does not occur, if at all, until the sheet has been so utilized.

Further, if the abrasive particles are regularly spaced and oriented, superior anchorage of the individual particles is obtained. This allows a high percentage of the abrasive particles to be fractured as they become slightly dulled, rather than to be removed by bond failure. The

.fractured particle exhibits an additional abrading face which will become effective as theabrading plane is lowered. As to why this surprising effect exists is not known; however, it is submitted, as a possible explanation, that the stresses set up in the bond and sizing adhesives (both in the drying and curing of the sheet and in abrading operations) are uniformly equalized and distributed thus necessitating a large force to dislodge the individual grains. To achieve this effect I prefer to have adjacent particles within about two particlewidths apart; also, such placement substantially decreases the possibility of any ridging or scratching.

Collectively these features surprisingly impart a uniform rate of cut throughout the life of the sheet. This makes unnecessary a continued increase in machine or hand pressure to keep the rate from diminishing, an especially important fact in operations where operating pressure cannot readily be increased. Presumably, this uniformity of rate is due to the fact that the number of abrasive particles becoming effective as the sheet is used does not substantially increase as the abrading plane is lowered.

My flexible coated abrasive sheet is also suitable for use in refinishing trades, such as removing varnish and paints from fioors and the like, due to resistance to filling and clogging as a result of the regularly spaced open spaces between the individually spaced particles.

It may be observed in studying the present methods and apparatus that difficulty might be encountered in stripping free the masking web from the composite sheet material 26 Without pulling with it, i. e. offsetting portions of the binder adhesive or disengaging the abrasive particles as the particles are oriented. I have found such may be avoided by utilizing a masking web which resists adhesion thereto by the binder adhesive. For example, the masking material may be fabricated from materials which inherently resist such adhesion per se such as polytetrauoroethylene, marketed by E. I. du Pont de Nemours and Co. under the trademark Tefion." T effen" is characterized by its inertness to almost any chemical or physical reaction. A'woven masking web of the material might be produced by weaving extruded sintered strandsinto a clothlike structure wherein the nterstices are of a size and present in the pattern desired. The weave should be of a type that resists displacement of the elements so the pattern will not deviate substantially as the web is used and reused.

If a continuous perforated masking web is desired, a

dense film of polytetrauoroethylene may be sd utilized by punching holes of the desired size and in the* desired configuration.

Another example of a suitable adhesively inert mavoccurs when the adhesive is in the substantially undried form, sufficient drying must occur subsequent to the lami nation and application of grit and prior to the stripping operation to permit the adhesive to develop sufficient anchorage of the abrasive particles to withstand the separation without being wrenched out or jerked free of the adhesive. I prefer in most cases to allow a slight amount of drying on the surface of the adhesive prior to lamination. When the masking web is laminated to such a partly dried adhesive coated surface, negligible offset occurs and the abrasive particles become sufficiently adherently attached to carry out the stripping operation successfully. If the masking web is pretreated on the surface thereof"- with release agents exhibiting low affinity for the bond adhesives, several other materials may be satisfactorily employed as the masking web. Such materials as polyvinylidine chloride, currently marketed under the trade name Saran, and the polymeric condensation product of terephthalic acid and ethylene glycol presently known as Mylar are very suitable. Also, I find metal screens y woven and formed to the desired pattern, when suitably surface treated in necessary cases, serve as preferred masking webs. Woven open mesh cotton cloth which is waterwetted prior to lamination will satisfactorily serve as the masking web though disadvantageously it lacks the durability desirable for prolonged usage.

These and other masking webs may be treated withfi silicone polymers, tiuorocarbon liquids or polymers, or other release agents capable of remaining firmly bonded to the film or strands of the web while being non-adherent y to the abrasive binder.

Other types of surface treatments may be employed in rendering the masking webs adhesively non-receptive such as waxes (applied either by solvent or melt-'coating techniques) and "pseudo-film-forming materials such as mica filled methyl cellulose solutions. However, these vmaterials sometimes fail during stripping of the masking web from the adhesive coated composite sheet product and leave deposits on the latter. The deposits may deleterious- -ly affect the adhesion of the subsequently applied sizing adhesive coat. When these latter types of surfacing operations are carried out the masking web would be treated after it is removed from supply roll 16 sufficiently precedeut to the lamination step to allow the coating to harden.

As previously mentioned, the masking web may consist of either a continuous perforated film or a woven fibrous or filamentous member. I prefer to employ the woven web where the abrasive particles will be relatively close together and a perforated film where the abrasive particles are spaced relatively far apart. If the holes in the otherwise continuous film are to be located closer than about one diameter thereof apart, the web in most circumstances does not have sufficient strength'to withstand the stresses applied thereto during the course of the coating and stripping operation. On the other hand, a woven structure becomes more and more impracticable as. the apertures placed therein become farther apart. Also, I find some materials more applicable in one form than the other, Tetlon" and polyethylene being particularly suitable in film-form, whereas metal wire, glass filaments or threads, and cotton threads are preferred in the form of woven members.

Of particular note in my new .and novel invention is the fact that substantially each and every abrasive particle is oriented with the long axis thereof normal to the backing sheet as the masking web is stripped away from the composite sheet. When the operations' are carried out according to thc methods herein described, such oricntation is accomplished with removal of substantially no abrasive particles, thus the desired regular pattern of individually spaced oriented particles with regularly shaped spaces therebetween may beattained.

The size range of the apertures required in the masking web is dependent on the classification and size of the orientable abrasive particles. For a given size range of orientable abrasive particles, a corresponding web aperturesize is required which will permit passage of only a single particle at one time.

An alternative means of facilitatingy good anchorage of the abrasive particles in the individually spaced repetitive pattern desired while allowing for subsequent easy removal of the masking web and orientation of the particles,

without offsef of the binder adhesive or disengagement of the oriented particles involves the application of infra red or other radiant energy and conversion thereof to heat at the surface of a pre-coated thermoplastic adhesive surface. nature such that incident radiant energy will not readily be absorbed and a certain required amount of differential heating is attained when the web is in contact with nonretlective material. The energy is directed on the combination web just prior to particle deposition. The restricted coating areas will be heated due to absorption of the rays while the surface contiguous with the reflective masking web `does not rise to as high a temperature since the energy directed at the masking web is reflected and not absorbed. In such a ease the masking web would not be laminated to the backing web until after the solvents had been removed from the latter to the extent that it was in a non-tacky state. l flnd this method particularly applicable when heat advancing resinous adhesives are used since they exhibit thermoplasticity prior to curing.

Referring to Figure 5, wherein an abrasive sheet backing 40 runwound from storage roll 4l is coated on one side with a suitable binder adhesive 42 contained in trough 43 by means of binder applying rolls 44 and adhesive feed roll 45. The sheet is led through oven 46 to drive off the solvent and leave a smooth dry thermoplastic adhesive coating heated to a temperature 20-30 F. below the softening point thereof. The sheet is then led around contact roll 47 and combined by lamination on the adhesive face with reflective masking web 48, unf wound from supply roll 49. After being thus` joined, the composite of the still warm adhesive coated backing web 40 and the masking-web 48 is irradiated from radiant energy source 50 whichA directs radiant energy onto the composite web. The radiant energy reaching the expos-:d preheated adhesive surface is absorbed by the adhesive and converted into heat thereby raising the temperature of the adhesive above the softening point thereof; that radiant energy falling on the reflective masking web is reeeted and the temperature of the adhesive contiguous therewith remains below the softening point. The cornposite web is immediately then passed over electrostatic coating means 51.

Abrasive particles 52 are deposited by gravity from a hopper 53 (which may be fitted with an adjustable gate or outlet, not shown, to control the quantity of particles released) onto the upper flight of an approximately horizontal conveyor belt- 54 which is positioned to conduct particles 52 below the composite web into the electrostatic field generated by electrostatic coating means 5l. whereby thc abrasive particles 52 are propelled upwardly against the composite web and a portion are brought into adhering contact with the softened adhesive of the restricted `abrasive coating areas. Upon being contacted with the abrasive particles, the temperature of the ad- In this method, the masking web is reflective in lli hesive drops below the softening point, and the particles thereby become sufficiently anchored in the adhesive to effectively resist detachment during the subsequent stripping operation. The `surplusl abrasive particles not adhering to the web are collected in hopper 55.

The abrasive coated composite web is led to stripping means (here shown as guide roller or pullover drum 56 positioned adjacent to the combination web) where the reflective masking web 48 is pulled away from abrasive sheet material 58 and wound up on roll 57, thereby orienting the coated abrasive particles to a position with thc long axes thereof normal to backing sheet 40. The

abrasive coated sheet 58 is led around suitable guide rollers and nally dried and cured on racks 59 in oven 6l). thence the cured sheet 62 is wound up on storage roll 61 or directly sized and cured. The cured abrasive sheet 62 is similar to that of cured sheet 26 of Figure l and will generally be sized prior to use in abrading operations. I refer to this method utilizing radiant energy as the differential heating method.

The reflective masking member 48 may be composed of screens or films of low heat conductivity and previously rendered reflective by a metallized coating or other means. The radiant energy herein described may be derived using conventional radiant heat lamps or elements commercially obtainable which emit rays primarily in the infra red range.

Where it is desirable to make my new and novel abrasive structure using water-soluble animal or vegetable glues as the bonding adhesives, I find it desirable to dry the adhesive coated sheet at a temperature and for a time sufficient to just commence gelling the glue, then to laminate the masking web and deposit the abrasive particles on the combination web before the adhesive completely gells flexible coated abrasive sheet material, where a heatadvancing water-insoluble resinous adhesive is employed, 1 find the alternative differential heating method especially suitable.

It will be apparent that in the various method steps and apparatus for carrying them out, there are many equivalents which may be interchanged and placed in various combinations without departing from the spirit of the present invention. For example. deposition of particles onto the combination of adhesively coated backing and masking web may be by any method, such as by gravity. or by imparting kinetic energy to thc particles electrostatically, electromagnetically or mechanically, regardless of the type of backing and adhesive applied thereto and masking web used. Also` the masking web may be in thc form of an endless belt and led back around and reused in lieu 0f winding and unwinding from storage rolls.

The number of abrasive particles per unit area and the manner in which the lateral rows of individually, regularly spaced oriented particles repeat is not meant to be limited to the types and examples specifically de scribed herein, for such number and manner could vary widely in individual sheet and still be oricntably placed with regularly shaped spaces therebetween, and hence within the spirit hereof. l find the preferred number of abrasive particles per unit area for an abrasive particle size range for any given abrading operation at a given average pressure by experimentally varying the number of particles per unit area in experimental sheets over a range, measuring the amount of stock removal, plotting a curve of particles per unit area versus stock removal and determining the optimum location on thc curve.

The curve of Figure 6 is based on the results of a typical series of experiments. Abrasive discs were cut from experimental sheets having varying numbers of individually spaced oriented grit 24 aluminum oxide abrasive particles per square inch. The discs were rotated at an average surface speed of 2,856 surface feet per minute (1,750 R. P. M.) and brought into abrasive contact with However. in preparing a water-proof a diametral section of a l inch steel rod at a continuing force of pounds. The test continued until substantially no further stock rer: oval was observed, at which time the amount of stock removed by weight from the 4 steel rod was measured. The total amount of stock removed was-plotted on the ordinate and the number of particles per square inch on the experimental discs was plotted on the abscissa. Lines connecting the plotted points show the surprising peak in the curve over the preferred range of numbers of particles per square inch. As may be seen, the disc having 196 particles per square inch is at the peak of the curve for this particular series of experiments.

An abrasive sheet having this optimum number of oriented abrasive particles per square inch was prepared as described in the following example. In the example, the ingredients are given in parts by weight unless otherwise indicated. y

Example l A paper backing sheet previously treated with butadiene-acrylonitrile polymer was prepared for the application of the orientable abrasive grits by roll-coating with an abrasive binder coat consisting of a mixture of 100 parts of paraphenyl phenol-formaldehyde oil-soluble heatadvancing .resin and 300 parts of China-wood oil in a volatile organic solvent, the solution having 65 percent solids, at a wet coating thickness of 2-3 mils. After the binder coating had been allowed-to dry at C. for 1-2 minutes, a copper screen previously treated with a silicone polymer serving as a release agent and containing 196 equally spaced apertures per square inch was laminated to the coated face of the backing at a lamination pressure just sufficient to allow continuous contact over an 18 inch laminating drum. The apertures were of a size so as to allow only one abrasive particle to pass through at a time.

An excess of grit 24 aluminum oxide orientable abrasive particles were deposited by gravity on the combination backing and screen and the web was shaken by the action of a web beater so as to jostle individual particles into substantially every aperture of the screen. The excess particles were removed by gravity and additional shaking and thescreen was then stripped from the composite sheet at a low angle of separation by guiding the screen on a path slightly divergent from that of the composite sheet. The abrasive sheet was-then racked in festoons and dried and cured for about 24 hours in a 185-195 F. oven. The abrasive sheet of Example I is illustrated and described in Figure l and Figure 4 of the drawing as sheet 27.

A 60 percent solids phenol-formaldehyde resin sandsize coating was then applied over the abrasive layer Aat a wet coating weight of about 8 mils. The sheet was then fully and finally cured by heating in festoons for 10-24 hours in a 18S-195 F. oven, and was rolled up for storage and for conversion into sheets of desired size. The instantaneous rate of cut of the 196 particle disc was substantially uniform over the abrading life of the sheet and was atleast equal or superior to the maximum-rate of the standard closed coat prior art sheet having similar abrasive grit size. The amount of total stock removed `in the above described test was approximately four times l that removed by a typical full coated abrasive sheet prepared by prior art means when tested under the same conditions.

Example 1l The abrasive sheet 27, as illustrated in Figure l, and prepared as described Example I, was coated with a second grit-binder coat identical with the first ata wet coating weight of 4-6 grains per 4" X 6" sheet. An excess of grit 100 aluminum oxide labrasive particles was then randomly deposited by gravity on the coated face of the sheet and the excess mineral not adhesively attached was v30-35 grains per 4" x 6" sheet.

ure 7 wherein the individually spaced oriented abrasive particles .72, anchored in first abrasive binder layer 71 coated on backing 70, and the vexposed portions of said rst bonding layer 71 are coated with a second abrasive binder layer 73 having small abrasive particles imbedded throughout said layer.

The instantaneous cutting rate of the sheet of Example II, illustrated in Figure 7, was markedly superior to that of' Example l. Surprisingly, all the abrasive characteristics of the sheet were those of a grit 24 sheet rather than of a grit 10() sheet. The finer abrasive particles seem to prolong the effectiveness of the coarser particles by fracturing and effectively resharpening the dulled particles. Such a result occurs when the second abrasive particles are relatively small when compared with the larger orientable particles. I find that the average classification screen aperture size of the smaller particles should be V6 or less with respect to that of the larger particles.

Example III In a modification of Example II, an unsized abrasive sheet made as described in Example I and vconsisting of vertically oriented regularly uniformly individually spaced abrasive particles bonded to a fiexible backing with an abrasive binder was first prepared. The binder was hardened to a point at which the abrasive particles were iirmly held in position during subsequent operations. A second coating of tiuid abrasive binder was applied over the abrasive-coated surface, and abrasive grains were then randomly applied. The sheet was further cured, coated with a light sand-size, and finally completely cured. The amount of binder and the particle size of the randomly applied grain were such as to cause only partial filling of the free spaces between the particles of the first abrasive coat. The upper portions of the oriented particles of the first abrasive coat remained exposed and in position for effective abrasive action. The substantially smaller and non-oriented particles of the second abrasive coat served to support and maintain the oriented particles in position against the most violent stresses encountered in abrading operations.

The particles of the second coat were of the same abrasive material as those of the first coat in this example, but were of considerably smaller size and of a more blocky shape characteristic. However, since the second coat is effective primarily as a reinforcing layer rather than an abrading layer, the particles may be of any suitable reinforcing material rather than of the same chemical 'composition as the primary abrasive particles. Thus, orientable silicon carbide abrasive particles may be supported in position by addition of substantially smaller particles of silica, crushed rock, glass cullet or sphericles, crushed fully cured phenolaldehyde resin, or metal particles, by the procedure indicated.

The second binder coating may be eliminated, if desired, by applying the second particulate coating after orientation of the individually spaced abrasive particles, either before the first binder coat has been hardened or after temporarily reactivating the exposed areas of said first binder coat byvapplication of solvent or by irradiation with absorbable radiant energy. A first binder coat of increased thicknessover that described in Example I is preferred. Such a procedure permits the addition of abrasive-supportingv particles between the abrasive grits without covering the top exposed surfaces of the grits. A sizing coat completes the structure.

Having now described various embodiments of my invention, it is to be understood that the scope thereof is not` to be limited thereto, but rather to be taken from the specification as a whole, including the appended claims.

I claim:

1. A method for producing a exible coated abrasive sheet havingv elongated abrasive particles regularly uniformly individually spaced in an oriented position thereon comprising coating a flexible backing sheet with a layer of a hardenable grit bonding adhesive, placing in said adhesive elongated abrasive particles in individual spaced relation with regular spacing therebetween, orienting said particles to a position such that the long axes thereof are substantially normal to said backing, and hardening the adhesive while said particles remain oriented.

2. A method for producing a tlexible coated abrasive sheet having elongated abrasive particles regularly uniformly individually spaced in an oriented position thereon comprising coating a flexible backing sheet with a layer of a hardenable grit bonding adhesive, placing in saidA adhesive elongated abrasive particles in individual spaced relation with regular spacing therebetween, permanently orienting said particles to a position such that the long axes thereof aresubstantially normal to said backing, applying a further coating of particles on the abrasive coated faceof the sheet, and hardening the adhesive.

3. The method of claim 2 wherein the particles of the further grit coating are no larger than about one-sixth the size of the regularly spaced oriented particles of the rst coating.

4. A method for producing a tlexible coated abrasive sheet havingelongated abrasive particles regularly uniformly individually spaced in an oriented position thereon comprising coating a flexible backing sheet with a layer of a' hardenable grit bonding adhesive, placing in said adhesive elongated abrasive particles in spaced relation with regular spacing therebetween, permanently orienting said particles to a position such that the long axes thereof are substantially normal to said backing, applying a further coating of hardenable grit bonding adhesive and abrasive particles on the abrasive coated face of the sheet, and hardening the adhesive coatings.

5. Themethod of claim 4 wherein the particles of the further grit coating are no larger than about one-sixth the size of the regularly spaced oriented particles of the first coating.

6. A method for producing a flexible coated abrasive sheet having elongated abrasive particles regularly uniformly individually spaced in an oriented position thereon comprising coating a flexible backing sheet with a layer of a hardenable grit bonding adhesive, temporarily laminating a masking web having a repetitive. series of apertures placed therein on the adhesive face of said sheet in superimposed relation therewith, applying elongated abrasive particles to the web-covered surface of said sheet, causing single abrasive particles to pass through said apertures and to be adhered in said adhesive, removing excess particles, stripping said web away from said adhesive and said particles whereby said particles are oriented with the long axes thereof being positioned essentially normal to said backing sheet, and hardening said adhesive while the particles adhered therein remain oriented; said apersheet, an abrasive binder coat adhered to one face of said` sheet, a layer of elongated abrasive particles embedded in said binder coat and adherently attached thereto such that substantially all of said abrasive particles are individually positioned in spaced relation and in regular pattern leaving regularly shaped open spaces between said particles, said particles being oriented with the long axes thereof substantially normal to said backing sheet.

8. The article of claim 7 wherein the individually spaced oriented abrasive particles are substantially equidistant from each other.

9. A new and improved abrasive article of the nature of a flexible coated abrasive sheet having an extended uniform high rate of cut comprising a exible backing sheet, a first abrasive binder coat adhered to one face of said sheet, a layer of elongated abrasive particles embedded in said binder coat and adherently attached thereto such that substantially all of said abrasive particles are positioned in spaced relation and in regular pattern leaving regularly shaped open spaces between said particles, said particles being oriented with the long axes thereof being positioned substantially normal to said backing sheet, and additional hard solid particles bonded to the abrasive binder between said oriented particles and lying well below the plane of the exposed tips of said oriented particles.

10. The article of claim 9 wherein the additional hard solid particles are no larger than about one-sixth the size of the regularly spaced oriented abrasive particles.

11. A new and improved abrasive article ofthe nature of a tlexible coated abrasive sheet having an extended uniform high rate of cut comprising a flexible backing sheet, a first abrasive binder coat adhered to one face of said sheet, a layer of elongated abrasive particles embedded in said binder coat and adherently attached thereto such that substantially all of said abrasive particles are positioned in spaced relation and in regular pattern leaving regularly shaped open spaces between said particles, said particles being oriented with the long axes thereof being positioned substantially normal to said backing sheet, a second abrasive binder coat over said first binder coat and oriented abrasive particles, and additional abrasive particles embedded n said second binder coat.

12. The article of claim 11 wherein the additional abrasive particles are no larger than about one-sixth the size of the regularly spaced oriented abrasive particles.

References Cited in the le of this patent UNITED STATES PATENTS 

1. A METHOD FOR PRODUCING A FLEXIBLE COATED ABRASIVE SHEET HAVING ELONGATED ABRASIVE PARTICLES REGULAR UNIFORMLY INDIVIDUALLY SPACED IN AN ORIENTED POSITION THEREON COMPRISING COATING A FLEXIBLE BACKING SHEET WITH A LAYER OF A HARDENABLE GRIT BONDING ADHESIVE, PLACING IN SAID ADHESIVE ELONGATED ABRASIVE PARTICLES IN INDIVIDUAL SPACED RELATION WITH REGULAR SPACING THEREBETWEEN, ORIENTING SAID PARTICLES TO A POSITION SUCH THAT THE LONG AXES THEREOF ARE SUBSTANTIALLY NORMAL TO SAID BACKING, AND HARDENING THE ADHESIVE WHILE SAID PARTICLES REMAIN ORIENTED. 